As is known in the art, it is frequently desirable to fabricate, and electrically interconnect, both passive and active microwave components on a common substrate. Such arrangement is commonly referred to as a monolithic microwave integrated circuit. Typically the components are electrically interconnected with microstrip or coplanar waveguide transmission lines.
As is also known in the art, a monolithic microwave integrated circuit (MMIC) includes a semiconductor substrate having formed on a surface thereof electrically active and passive devices electrically interconnected with microwave transmission lines as described in, for example, U.S. Pat. No. 6,545,563, issued Apr. 8, 2003, Inventor Irl W Smith; entitled “Digitally controlled monolithic microwave integrated circuits”. Air-bridge structures are sometimes used in these MMICs. For example, many active devices are formed with inter-digitated electrodes. One such active device is described in U.S. Pat. No. 4,456,888, issued Jun. 26, 1984 and entitled “Radio Frequency Network Having Plural Electrically Interconnected Field Effect Transistor Cells”, assigned to the same assignee as the present invention.
As is also known, dielectric and protective coatings are often applied to MMICs to prevent mechanical damage or deterioration of the circuit elements from environmental exposure. The coatings must have low dielectric constants in order to prevent loss of performance of the circuit.
The inventors have recognized that while these dielectric coating may be required to cover and thereby protect the passive devices and interconnection microwave transmission line, for active devices, such as the FETs which use air bridges to connect circuit elements together, it is often found that even low dielectric constant films cannot match the performance of air under the air bridges. This impact is greatest on active transistor elements where the change in parasitic capacitance due to the change in the dielectric constant between the gate and air bridge directly impacts circuit performance.
Previous methods for removing the dielectric coatings from these the air bridges, including the region below the air bridges, included the use of a negative photo-imageable coating where the film is unexposed in the air bridge region and subsequent removed by development. Alternatively the dielectric coating could be masked with an etch-resistant coating except for the air bridge regions and an etchant is used to remove dielectric film from the air bridge regions. Both of these approaches fail to completely remove the dielectric film from under the air bridge, and extending the developing of the photo-imageable layer, or the etch time of the etched layer causes dielectric film loss in the areas where the film is required. Etching of the dielectric coating can also lead to undesirable removal of other inorganic dielectric coatings such as oxides and nitrides which can cause changes in the performance of the circuit.